1. Field of the Invention
The invention relates to ultra high purity gas analysis. More particularly, it relates to the calibrating of highly sensitive gas analysis equipment.
2. Description of the Prior Art
High purity gas applications, gas mixture standards are typically provided in cylinders at the parts per million (ppm) level. The semiconductor industry, however, requires ultra high purity (UHP) levels for gases used therein. This requirement arises from the need to decrease the dimensions of the line spacing for semiconductor devices. As the lines on semiconductor devices move closer together, impurities must be maintained in the low parts per billion (ppb) and even parts per trillion (ppt) range. The need for gas supplies with purity levels less than 1 ppb has driven industry to develop new analytical techniques for measuring gas purities. Recent advances in the gas analysis instrumentation art have provided users with the ability to examine lower concentration levels of impurities in main gas streams. The need to calibrate gas analysis instrumentation in the same range of impurities as would be found in the typical sample gas has led to increased demand for low concentration gas mixture standards. This need has occasioned the problem of obtaining a reliable gas standard for the calibration of such instrumentation. Gas mixtures supplied in cylinders or containers at the ppb or ppt levels are difficult to prepare and the reliability of the concentration is questionable. The reliability of a low concentration gas mixture is subject to question because of reactions occurring between the trace components of the mixture and the walls of the container. The adsorption/desorption of impurities onto all surfaces that come into contact with the gas presents a problem in measurements at low concentration levels. Such surfaces include regulators, valves, mass flow controllers, and transfer tubing runs, which deliver the calibration or sample gas to the analyzer. While it is difficult to know when an equilibrium has been reached with these components, this equilibrium is essential in order for the purity level of a gas to be confidently specified. Reactions can also occur at wetted surfaces, which can convert one impurity molecule into another, e.g. CO into CO.sub.2.
The transfer of gases from one container to another also introduces an uncertainty in this regard. For instance the possibility of atmosphere leakage is present when such a transfer takes place. The measured weight of an impurity gas becomes vanishingly small when trying to produce low concentration standards in cylinders, and the impurity level of the balance gas plays an increasing role in determining the final concentration.
To overcome these problems, a gas delivery system must have the ability to deliver multilevel calibration gas and a sample gas without major disruption. The system should be simple in nature, with the lowest transfer volume and minimum number of components, and uncomplicated to operate. Such a system and the related method of calibration are needed to achieve the ever tighter specifications in the electronic gas industry.
It is an object of the invention to provide an improved calibration system and method for ultra high purity gas analysis.
With this and other objects in mind, the invention is hereinafter described in detail, the novel features thereof being particularly pointed out in the appended claims.